This invention relates generally to systems for bridging network failures in intermediate voltage converters, and more particularly, to a system wherein brief network failures are bridged by reconverting kinetic energy which is stored in load motors into electrical energy by lowering the frequency of the intermediate voltage converter.
As used herein, intermediate voltage converters are understood to mean converters of the type which contain controlled rectifiers, or DC control elements, in their inputs. Moreover, such intermediate voltage converters may be provided with pulse width-modulated inverter control.
In several known machines, illustratively of the type which are used in the chemical fiber and glassware industries, machine operation must be maintained continuously even in the event of a brief network failure. It is particularly essential that the drives which are supplied electrical energy by the intermediate voltage converters do not become desynchronized during the period of the network failure.
U.S. Pat. No. 3,705,340 describes an illustrative known system which is provided with a capacitor buffer in the intermediate converter for energy bridging in the event of brief network failures. The system described therein is capable of bridging network failures which have a duration in excess of 500 milliseconds. Although such buffer arrangements have proven themselves to be praticable, it is a disadvantage of such systems that the intermediate converter voltage may be decreased by only a small amount during a network failure before the motors supplied thereby reach their stalling torque. Moreover, such capacitive storage systems are relatively expensive.
Network failures of longer duration may be advantageously bridged by kinetic buffering systems which, for example, contain a corotating generator connected to the network, or by utilizing the kinetic energy stored in the driven motors and the load. A technical prerequisite for such kinetic energy storage systems is that the work-performing machine and its drive system, must have a substantial moment of inertia. Moreover, means must be provided for lowering the frequency of the system during network failure.
British Pat. No. 1,085,520 illustrates a prior art arrangement wherein the frequency of the system is lowered in accordance with a predetermined time function. This system, however, has the disadvantage that the empirically determinable lowering of the frequency produces impermissible conditions if the moment of inertia changes, such as by switching the motors on and off. In arrangements wherein the motor speeds are controlled at different speed levels, the coupling of the generator feedback must be precise. For example, if the generator feedback of the motors is too light, the converter will be disconnected for lack of energy. On the other hand, if the generator feedback is too heavy, there is presented a danger of energy overloading which may cause the converter to be shutdown by its protective devices. As a result of these disadvantages, kinetic buffering cannot be widely used.
It is, therefore, an object of this invention to provide a kinetic buffering system which operates reliably over a wide range of network failure periods.